This invention relates to a method for cleaning a steam generator of so-called U-type, which steam generator consequently comprises an upstanding outer casing, a pipe bundle mounted therein, which extends in the shape of a reversed U, and a bottom which closes at the foot of the pipe bundle the openings between the bundle pipes and between said pipes and the outer casing, which steam generator is provided with at least two accesses, lying above the bottom and substantially opposite one another, to the inner side, through which said pipe bundle may be reached, method according to which through at least two accesses lying substantially opposite one another, a high-pressure lance which is movable along the lengthwise axis thereof, is rotatable about the axis thereof and bears at the end thereof a spray head unmovably mounted thereon with spray openings directed cross-wise to the lance axis, is brought between the pipes of said pipe bundle, and through at least one access, at least one suction line is brought down about next to the bottom adjacent the outer casing, the spray heads are simultaneously intermittently shifted between about the generator center and about next to the outer casing, in such a way that between said shiftings, the spray heads may spray between rows of the pipe bundle, the spray heads are rotated about the lance axis at least between said shiftings, liquid under a pressure higher than 200 bars is sprayed through said spray heads at least during such rotation, and the used cleaning liquid is sucked away from the bottom through the suction line during the cleaning.
Steam generators of the above-defined kind are used in nuclear power stations. The heating fluid which is heated inside the nuclear reactor is lead through the pipe bundle and abandons heat to the fluid, generally water, which lies in the space about the pipes, whereby steam is generated. Said steam is used for driving turbines and after cooling, is returned back to said space.
Due to various chemical reactions, there is formed about the pipe bundle, an iron oxide, namely magnetite. Said iron oxide settles on the pipe bundle, the bearing plates for said pipe bundle, and mostly the bottom. At each maintenance procedure, generally yearly, of the nuclear power station, the iron oxide has to be removed. In connection with the difficult reachability of the steam generator and mostly due to the prevailing radio-activity, the steam generator cannot be dismantled for such cleaning, so that the removal of the sludge and thus more particularly the iron oxide, requires special methods. There is always made use therefore of accesses which are provided in the lower portion of the steam generator, above the bottom and which during the normal operation of said steam generator, open on lines or are closed by metering apparatus or covers. Through releasing one or a plurality of said accesses, it is possible to spray loose with a high-pressure lance having a spray head, the sludge from the bottom and possibly from other portions of the inner volume of the steam generator. The main problem does however lie in removing said sludge from the generator.
In known methods for cleaning a steam generator of the above-defined type, with the purpose of such removing, an additional amount of liquid is fed to the bottom during the spraying of pressurized liquid through the spray heads.
In the known method of the kind defined in this Application, which is disclosed in Belgian Pat. No. 889,706 (French Pat. No. 2,510,232 is equivalent), said additional liquid amount is fed by means of a sprinkling head which is brought above the bottom center.
When the steam generator is provided between the bottom and the lowermost pipe supporting plate, with four accesses, spraying is made under a pressure of about 250 bars by means of four high-pressure lances with spray head, which are brought-in through said four accesses. Said high-pressure lances are moved simultaneously stepwise from the center to the outer casing, and the lances are always rotated in the same direction over a plurality of revolutions about the axis thereof between the succeeding displacements.
The four spray heads have an outer diameter of but 8 mm and cannot spray enough liquid to insure good removal of the sludge from the bottom. Consequently there is caused during the spraying, a flowing over the bottom, from the center to the outer casing, by means of the sprinkling head which sprinkles low-pressure liquid on the bottom. The cleaning liquid and the sludge are sucked in two mutually opposite locations lying next to the outer casing.
When the steam generator also has two accesses opening above the lowermost supporting plate and lying opposite one another, there is first cleaned above the lowermost supporting plate with a high-pressure lance. Use is made therefore of but one high-pressure lance with spray head which is brought-in succeedingly through both accesses, each time being moved stepwise from the center to the outer casing, and being rotated always in the same direction over a plurality of revolutions about the lance axis between the steps. In spite of said spray head having a larger diameter than the above-mentioned heads, it cannot spray enough liquid on the supporting plate to insure good removal of the sludge therefrom , so that also during the cleaning of the supporting plate and those portions of the steam generator lying thereabove, additional liquid is sprinkled with the sprinkling head on the supporting plate.
To remove all the sludge from the bottom, it is necessary with this known method to spray moreover at the end of the cleaning, with spray guns, liquid under a pressure of 300 bars on the bottom, at least against the outer casing.
The use of a sprinkling head during the cleaning and of spray guns at the end, makes the method intricate and time-consuming. Said additional liquid feeding devices naturally also increase the volume and costs of the apparatus required for the application of the method.
Moreover it has been noted that in spite of sprinkling with the sprinkling head, sludge could still remain on the bottom. In certain locations on the bottom, a driving-back of liquid and consequently a limited liquid flow did in fact occur, because the liquid sprayed by the spray head or heads did oppose the flow of that liquid being sprinkled by the sprinkling head.
The invention has for object to obviate said drawbacks and to provide a method for cleaning a steam generator of the above-defined type which allows a useful removing of the sludge from the bottom in a simple and fast way, that is without an additional supply of liquid to cause a flowing over the bottom and possibly a spraying with spray guns at the end being necessary.